I noticed today that an export was going along smoothly then suddenly crawled to nearly a halt. Glancing around I just happened to notice that the hard drive the file was being exported to had only a few gigs left on it. Another case of someone putting something into the trash and failing to empty it.
After emptying the trash -and freeing up over 30 gigs of space- the export speed seemed to pick back up to what would be expected. I'm not 100% sure that was the cause, it could have just coincided with exporting (and rendering out since nothing was pre-rendered) a particularly graphic laden section or something.